1. The Field of Art
This invention relates to a raster scan image data display controller including means for reducing flickering and, in particular, to an apparatus for effectively reducing the flickering produced when a still image is displayed by means of interlaced scanning.
2. The Prior Art
A known system, such as a teletext system, uses a display controller which enables image data items stored in an image memory to be displayed on a raster scan-type interlacing display unit, such as a CRT. A receiving device of such a conventional teletext system extracts a teletext (-like) signal superimposed on a television signal, stores it in an image memory, reads out the character signal from the image memory and produces a corresponding still image on a display unit, such as a CRT.
A still image displayed by means of interlaced scanning, with a field frequency of 60 Hz (frame frequency of 30 Hz) will be explained below, by way of example, with reference to FIGS. 6A to 6C. In FIGS. 6A to 6C, L.sub.40 to L.sub.42 and L.sub.303 to L.sub.305 show scanning lines, noting that the scanning lines L.sub.40 to L.sub.42, indicated by a broken line, are formed at an odd-numbered field period and that the scanning lines L.sub.303 to L.sub.305, indicated by a solid line, are formed at an even-numbered field period. Blocks B.sub.1 to B.sub.3 show one pixel each and the image data items of the image memory are used for both the odd- and even-numbered field periods; for example, for the scanning lines L.sub.40 and L.sub.303.
In the aforementioned image display using interlaced scanning, two fields, i.e., the odd- and even-numbered fields, are vertically shifted a predetermined amount on the display screen to provide a one image plane. In practice, the scanning line for one of the two fields--for example, the even-numbered field--is displaced, as shown in FIGS. 6B and 6C, due to a variation, for example, in the deflection system of the display unit.
In FIG. 6B two scanning lines for displaying the corresponding image data item are located close to each other, with their image blocks overlapping on this pair of scanning lines. In this case there is no problem because of the presence of a Flickering-prevention effect. That is, the same image data items read out of the image memory are displayed in display areas indicated by b.sub.1 and b.sub.3 in FIG. 6B, in which case the frame frequency is 30 Hz. Since the display area indicated by b.sub.2, in FIG. 6B, is scanned by the even- and odd-numbered fields, flickering decreases, due to the frame frequency of 60 Hz.
In the case of FIG. 6C, on the other hand, the paired scanning lines are located away from each other, resulting in a flickering image, due to image fluctuation, and thus in poor image resolution. That is, blocks B.sub.1 of one pixel in FIG. 6C are separated by a lower brilliance display area C.sub.1. In dislay area C.sub.2, blocks B.sub.1 and B.sub.2, belonging to two different pixels, partially overlap, with the result that the different pixels are displayed overlapping in display area C.sub.2 with a repetition frequency of 60 Hz, and consequently, a flickering image occurs there.
The flickering occurring in the two directions, due to interlaced scanning, differs for every display device, and, in the conventional display controller, poor image resolution occurs due to the presence of the poorly-defined pixels, resulting in the appearance of prominent flickering, a serious visual problem.